1. Field of the Invention
The present invention relates to an apparatus for determining whether an ambient temperature is higher or lower than a predetermined threshold temperature.
2. Description of the Related Art
Electrical equipment has a defined temperature range in which normal operation is allowed. Thus, some electrical equipment contains a temperature determining apparatus which determines whether an ambient temperature is higher or lower than a predetermined threshold temperature.
In such electrical equipment, when the temperature determining apparatus determines that an ambient temperature exceeds a range in which its operation is ensured, forced termination of operation is performed for preventing a malfunction, or recording of histories is performed for checking user environments, for example. While the aforementioned temperature determining apparatuses include products which operate mechanically, the temperature determining apparatuses which operate electrically are generally utilized.
A prior art example of such an electrical temperature determining apparatus will be hereinafter described with reference to FIGS. 1 and 2. As shown in FIG. 1, a prior art temperature determining apparatus 100 comprises a resistance element 102, a diode 103, a reference voltage generating circuit 104, and a comparator circuit circuit 105.
Resistance element 102 and diode 103 are connected in series between an operating voltage Vdd for an electrical equipment and the ground. Reference voltage generating circuit 104 generates a reference voltage Vref from the operating voltage Vdd which is lower than the latter. One input terminal of comparator circuit 105 is connected with the junction between resistance element 102 and diode 103, and the other input terminal of comparator circuit 105 is connected with the output of reference voltage generating circuit 104.
Resistance element 102 has a predetermined temperature-resistance characteristic in which a resistance value varies with a change in an ambient temperature. Thus, a change in the ambient temperature causes a change in a divided voltage VD generated by resistance element 102 and diode 103 from the operating voltage Vdd. The values of Vdd and Vref are set and the temperature-resistance characteristic of resistance element 102 is selected such that the divided voltage VD is equal to the reference voltage Vref at a predetermined threshold temperature.
Comparator circuit 105 compares the divided voltage VD with the reference voltage Vref and outputs a signal indicative of whether the ambient temperature is higher or lower than the threshold temperature.
As shown in FIG. 2, the reference voltage Vref is constant in a range "a" in which the operating voltage Vdd exceeds Vdd0. Furthermore, when the operating voltage Vdd is changed depending on the specification of the electrical equipment or the like, the reference voltage Vref is also changed in accordance with the operating voltage Vdd even if the ambient temperature is constant. As a result, the range of the operating voltage Vdd in which a temperature can be determined, is limited to, for example a range "b" in FIG. 2.
Additionally, since the reference voltage Vref generated by reference voltage generating circuit 104 is temperature-dependent, the reference voltage Vref varies with a change in the ambient temperature, independently of the divided voltage V0. As a result, it is difficult for the temperature determining apparatus 100 to accurately determine whether an ambient temperature is higher or lower than the threshold temperature.
It is obvious that the aforementioned problems similarly occur in the temperature determining apparatus with a second resistance element in place of diode 103.